The constructor of WireBytesRef checks that offset+length is still in
the uint32_t range. This CL avoids triggering this check on illegally
size strings.

R=ahaas@chromium.org
BUG=chromium:734246

Change-Id: Iab5c7013aa3e0ac5060bc4733e712a1652679b1a
Reviewed-on: https://chromium-review.googlesource.com/539402Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46050}